Duplicate Key

I don't think so. I would just create the giveaway, and if the key doesn't work, send the second key. If you go to the winners page after the giveaway ends, you can just replace the bad key with the good one and then send the winner a message to try the new key. As long as you eventually get a working key to the winner, everything will be fine.

Some games (but not all) have an option to view the CD key. Highlight the game and look at the right side of Steam's browser. There may be a link that says CD Key on it. Not all games have this feature.

Otherwise, just create the giveaway. If the winner says the key doesn't work, give him the other key. It's easy enough to change the key after the giveaway ends.

Just say this is the case in the giveaway description. Take your best guess which is used and be on the lookout for a message from the winner of it doesn’t work. You can edit keys after creating the giveaway or I think after you send the key so it should be pretty easy to get the winner the correct key.